Zoroaster (the Greek name by which the Iranian prophet Zarathustra has become known) is traditionally believed to have lived and taught in the early part of the 6th century BC. His home is probably in the region to the east of the Caspian Sea.

The main theme of Zoroaster's teaching is to replace the numerous ahuras or gods of the traditional Indo-Iranian religion with just one ahura, the supreme God or 'Wise Lord', Ahura Mazda.

Zoroaster's original concept of Ahura Mazda is found in what are believed to have been his own discourses, the Gathas, which form the opening section of the Avesta, the holy book of Zoroastrianism. Ahura Mazda, he says, created a pair of twin spirits as his sons. One, Spenta Mainyu, chose truth, light and life; the other, Angra Mainyu, preferred deceit, darkness and death.

As the religion of Zoroastrianism develops, after its founder's death, Ahura Mazda himself (sometimes now shortened to Ormazd) tends to take over from his good son the responsibility for thwarting Angra Mainyu.

Human history, according to Zoroastrian belief, reflects the struggle between these eternal opposites, Ahura Mazda and Angra Mainyu, good and evil, light and darkness, truth and deceit. Zoroaster rejects all but one of the forms of sacrifice practised by the Indo-Iranians, and keeps only sacrifice by fire. So fire becomes, in Zoroastrianism, the sacred symbol of truth.

It is not known how the religion spreads through Iran to become, eventually, the official religion of the Achaemenid dynasty. But by about 500 BC Darius I is proudly proclaiming in his inscriptions: 'By the grace of Ahura Mazda I am king; Ahura Mazda gave me the kingdom.'

Wise men of the east

The Magi, a priestly sect (whose name has given us 'magic'), soon become associated with Zoroastrianism - very probably making a politic adjustment from the service of an earlier Iranian religion.

The power of Persia gives the Magi a special status; they are the wise men of the east. As such, five centuries later, they arrive to add weight and authority at the birth scene of another religion - Christianity.

The Magi, who in the Christian story bring gifts to the infant Jesus, travel from a Persia ruled by the Parthians, in origin a dynasty of nomads. But the region has been culturally under the influence of Greece ever since the conquests of Alexander the Great.

The Greeks are tolerant of other religions, and the Parthians adopt much of Zoroastrianism - even erecting fire altars in honour of Ahura Mazda. So the religion survives, though not with the status which it enjoyed under the Achaemenids. The situation improves dramatically with the fall of the Parthian dynasty, and the rise of the Sassanians, in the 3rd century AD.

The Sassanians: 3rd - 7th century AD

The founder of the Sassanian dynasty, Ardashir, has strong links with the ancient Persian religion. His father is in charge of a temple to Zoroaster in the region of the ruined Persepolis before he kills the local ruler and takes his place. Ardashir inherits this petty kingdom and enlarges it - by defeating and killing local princes - until he is in a position to be crowned king of Fars in about 208.

A continuous process of slow expansion, at the expense of the Parthians, brings him to Ctesiphon. He enters the Parthian capital in triumph in about 224 and is crowned 'king of kings'. The new king is proud of one particular ancestor, Sassan; his dynasty becomes known as Sassanian.

Near Persepolis, at Naqsh-e-Rustam, Ardashir commissions a great relief sculpted high in the rock face. It depicts him on horseback, with a dead Parthian beneath his horse's hooves, while he receives the royal crown from Ahura Mazda.

With the restoration of the first authentically Persian dynasty since the Achaemenids, the cult of Ahura Mazda becomes again the official state religion. There is now a ritual hierarchy throughout the empire, with chief priests for each major district and a supreme priest wielding overall authority.

In the Sassanian period the main features of Zoroastrian ritual become established. The most important duty of a priest is to maintain the sacred flame of Ahura Mazda. Elaborate ceremonies are carried out to ensure its purity; the priest's mouth is even covered to avoid pollution from his breath. On important holy days, such as Nauruz (the Persian new year), the fire is displayed on a rock or high place to each community.

The need for purity also lies behind the Zoroastrian treatment of the dead. Corpses are exposed on mountain tops, to vultures or wild dogs, until the bones are clean and dry for burial. Only in this way can earth, water or fire be preserved from pollution by dead human flesh.

Zoroastrians and Parsees: from the 7th century

For three centuries after the Muslim conquest of Persia, Zoroastrianism remains of importance in the region. But gradually the majority of Persians convert to the religion of the new ruling caste, whether for reasons of conviction or convenience. A minority of Zoroastrians seek greater liberty elsewhere. They move to India, where they establish themselves in Gujarat as the Parsees (the Persian word for 'Persians').

A few Zoroastrians remain in Iran, to be found even today in the remote desert cities of Yazd and Kerman. They have been known to Muslims until recently as gabar, probably a version of the Arabic kafir ('infidel').